The invention relates to a process to electrically excite a laser gas, especially a CO.sub.2 -He-N.sub.2 mixture, which is admitted at an angle, preferably at a perpendicular angle, to the axial laser gas discharge gap, and which is ignited by means of bunched microwaves.
Laser light is often generated by means of light reinforcement from stimulated emission in an optical resonator, consisting of at least two mirrors and a laser-active medium. The laser-active medium is made up of excited atomic systems; in the case of the CO.sub.2 laser, it consists of CO.sub.2 molecules.
The excitation often occurs by means of an electric discharge. When this discharge is ignited, the electric field strength inside the discharge tube must take on substantially higher values than those that are necessary to maintain the discharge plasma. When microwaves strike the laser gas which has not yet been excited, the laser gas ignites if the field strength is sufficient, thus creating a plasma zone. This plasma zone absorbs the microwaves, additional electrons are formed and the plasma zone expands until, at a certain electron density, the so called cut-off density, almost all of the microwaves are reflected by the plasma in the direction of the microwave transmitter. In this process, the electric field strength between the transmitter and the plasma grows, and the plasma expands further in the direction of the microwave transmitter. This process continues until the microwaves have reached the wall of the tube or the microwave inlet window.
The cut-off density, which is important for the onset of reflection, is a function of the microwave frequency and the collision frequency between electrons and molecules. When this cut-off density is reached, a final state is achieved in which the microwaves are totally absorbed in the wall boundary layer and can no longer penetrate into the discharge space. The wall boundary layer heats up more and more, often causing damage to the dielectric discharge tube and to the microwave window.
It can be seen in the publication, "Schock, W., Laser Symposium 1985, 13 DFVLR (West German Research and Experimental Institute for Aviation and Aerospace) Institute for Technical Physics" that a highly absorbent wall boundary layer with a high electron density forms in the discharge gap of gas lasers during microwave excitation, normally rendering the laser operation inefficient. In order to avoid the wall boundary layer, the West German Research and Experimental Institute for Aviation and Aerospace (DFVLR, Institute for Technical Physics) has taken the approach of bunching the microwaves in a jet flow with a high pressure differential. As a result of the build-up of high pressure behind the dielectric window, ignition in this area is avoided. The ignition of the laser gas occurs in the low-pressure range behind the jet. A maximum continuous CO.sub.2 -laser power of 340 W, with an efficiency rate of 7%, can be achieved with a microwave power of 4.75 kW. Since the laser gas flows in the propagation direction of the microwaves and since the resonator is perpendicular to the laser-active medium, which is being formed non-homogeneously, and the resonator only comprises a part of the medium, the efficiency rate of this arrangement is low. The entire system is very complex and expensive because of the large mass flow required and because of the high pressure differentials.
In the "Journal of Applied Physics" 49 (7) July 1978, an article titled "Laser-generation by pulsed 2.45 gHz microwave excitation of CO.sub.2 " by Handy and Brandelik, pages 3753 to 3756, describes a process for the microwave excitation of a gas laser, which produces a gas laser of a similar class. With this gas laser, the microwaves penetrate the laser gas perpendicular to the flow of the laser gas, which flows into a discharge tube inlet positioned at a perpendicular angle to the microwave buncher and which flows out at a discharge tube outlet positioned perpendicular to the microwave buncher. On the basis of this configuration of the arrangement, the heated-up plasma collects along the dielectric discharge tube wall and forms a highly absorbent wall boundary layer. This leads to a low rate of efficiency of the gas laser and requires a cooling system with nitrogen that is pre-cooled to 200 K.